The present invention relates generally to an improved joint for joining clad materials and, in particular, to the joining of clad materials in which the base material and cladding material are galvanically incompatible for use in a corrosive environment.
The cladding of ferrous-base alloys with other metals having properties desired for particular application is much practiced in the art since ferrous-base alloys offer a relatively inexpensive base material which has good structural and fabrication properties. Thus, carbon steel has been clad with metals such as aluminum or titanium which provide a high corrosion resistance. One especially attractive combination is carbon steel clad with titanium. Pure titanium has outstanding corrosion resistance in environments such as plain water, sea-water, or brackish water, but, compared to carbon steel, is an expensive material which also does not satisfy the strength requirements for some applications. On the other hand, carbon steel is relatively inexpensive compared with titanium and possesses mechanical properties which are desirable for many structural components, but can be destructively attacked by corrosive environments. When a base of carbon steel is clad with titanium, the resulting clad material has the beneficial structural properties of carbon steel and the corrosion resistance properties of titanium. Similarly, aluminum has beneficial corrosion resistance quantities which, when combined with a base of carbon steel, provides the dual benefits described above in relation to the carbon steel-titanium clad materials. Beside titanium and aluminum nickel, copper, and tantalum have been used in conjunction with carbon steel bases in many applications. U.S. Pat. No. 3,464,802 issued to Meyer on Sept. 2, 1969 and U.S. Pat. No. 3,125,805 issued to Horrigan on Mar. 24, 1964 describe and illustrate the use of a corrosion resistant metal as a cladding on a carbon steel base.
In many cases the cladding material is explosively welded to the base material in a clad material assembly. The process of explosively welding dissimilar metals together is described in U.S. Pat. No. 3,614,827 issued to Knop et al. on Oct. 26, 1971 and U.S. Pat. No. 3,735,476 issued to Deribas et al. on May 29, 1973. Explosive welding has been found to be an advantageous way of metallurgically bonding two dissimilar metals because, generally, the metals cannot be easily welded together with the use of conventional methods because brittle zones of intermetallic alloys are formed due to diffusion during heating. These zones can result in the formation of cracks when exposed to thermal or mechanical stresses which could possibly result in the rupturing of the weld bond. This same problem also occurs when attempting to weld two explosively clad metal plates together. In apparatus which comprises explosively clad metal plates, certain difficulties are often encountered when welding the metal plates together or when welding tubes or pipes to the clad metal plates. This is true, in particular, if the two metals bonded by the explosive cladding operation can form intermetallic compounds or other weakened compounds as in the case of steel bonded to aluminum, molybdenum, tantalum, titanium, niobium, tungsten, palladium or zirconium or in the case where copper is bonded with aluminum. If the welding heat introduced into the bonding zone is too high, brittle intermetallic substances can be formed by diffusion and can lead to a detachment of the cladding layer at the locations affected by the heat. This results in brittleness of the weld seam and severely reduces its corrosion resistance.
Two major problems must be overcome when clad metallic plates are used in construction. First, the plates must be made in such a way that they are able to be joined to other members without creating the brittleness, described above, which can occur when two dissimilar metals are welded together. Secondly, in the regions near the edge of clad metal plates, the two metallic compounds can react destructively when they are disposed proximate each other in a corrosive environment such as seawater. This is caused by the galvanic incompatibility of the two materials and results in the destruction of one or both of the materials. For example, when aluminum and carbon steel are placed in close proximity to each other in a seawater environment, the aluminum dissolves in order to galvanically protect the steel. Similarly, when carbon steel and titanium are combined in a corrosive environment, the carbon steel is dissolved to galvanically protect the titanium. When aluminum and copper are combined in this type of environment, the aluminum is attacked by the copper and when titanium and copper are placed in close proximity within a brackish solution, deep pitting of the copper occurs.
In heat exchangers in which brackish or other saline water is used, this problem becomes acute. Since the tubing of a heat exchanger accounts for a significant portion of the overall cost of the heat exchanger, it is economically advantageous to manufacture the tubing from a material which provides the desirable heat conductivity characteristics at a minimal cost. In many cases, the most desirable material is aluminum. Since the tubes extend through and are welded to a tube sheet, it is necessary that the material of the tube and the material of the tube sheet are compatible since they will be existing in close proximity to each other within a highly corrosive environment such as saltwater and since they generally must be welded together. This application calls for aluminum tubes to be inserted through a tube sheet which has an aluminum surface. Since a tube sheet which is made entirely from aluminum would be prohibitively expensive and would not possess the required structural characteristics, the tube sheet can be made of carbon steel as a base material which is clad with aluminum. Therefore, this is a typical application in which the problems concerning dissimilar metals within corrosive environments and the welding of dissimilar metals, as discussed above, can readily occur.
The present invention provides a clad metal plate which comprises a base member which is made of a material with good structural characteristics, such as carbon steel, a cladding layer made of a material with good corrosion-resistant characteristics, such as titanium or aluminum, and an intermediate layer disposed between the base member and the cladding material in areas where the base and cladding materials would normally be proximate each other within a corrosive environment such as along the edges of the clad plate. The intermediate layer can be any material which is weldably compatible with the base material and does not react destructively with the cladding material when placed in a corrosive environment. When the base material is carbon steel and the cladding material is either titanium or aluminum, the intermediate layer can be Inconel 600, Inconel 625, 304 stainless steel or 316 stainless steel among others.
The intermediate layer of a clad plate made in accordance with the present invention can be disposed in a recess in the base material so that the top layer of the intermediate material is generally coplanar with that of the top surface of the base material. It can be either explosively welded or otherwise clad to the base material or disposed within the recess as a weld deposit. The cladding layer can then be explosively or otherwise welded to the base material and intermediate layer with the edges of the cladding material being recessed from the edge of the plate so that the cladding material is not proximate the base material along its edge. When two plates made in accordance with the present invention are welded together along their edges, the carbon steel base members can be welded together using conventional welding techniques. The respective intermediate layers of the two plates can then be welded together using a weld deposit which is compatible with the intermediate layer and which does not form brittle zones between the plates or regions which have reduced corrosion resistance. Since the cladding layers of each of the respective plates are recessed from the welded edge, they need not be metallurgically joined to each other.
The present invention provides a construction by which clad metal plates can be constructed in a way that avoids proximity of dissimilar metals which, in a corrosive environment such as seawater, would otherwise react destructively with each other due to their galvanic incompatibility.